Direct electrical heating arrangement with a power electronic converter

ABSTRACT

An arrangement provides an AC current to a load for direct electrical heating. The arrangement includes a AC-DC-AC converter cell. The converter cell has at least two converter input terminals connected to at least two transformer output terminals. The converter cell has a first converter output terminal and a second converter output terminal, wherein the first converter cell output terminal is adapted to be connected to the load.

RELATED CASES

The present patent document is a divisional application of U.S.application Ser. No. 14/237,062, filed Feb. 4, 2014, which is a § 371nationalization of PCT Application Serial Number PCT/EP2012/065511,filed Aug. 8, 2012, designating the United States, which is herebyincorporated by reference. This patent document also claims the benefitof EP11176835, filed Aug. 8, 2011, which is also hereby incorporated byreference.

FIELD

The present embodiments relate to an arrangement for providing an ACcurrent to a load for direct electrical heating of a pipeline portion,and to a pipeline heating arrangement.

BACKGROUND

Direct Electrical Heating (DEH) of subsea pipelines for gas productionis a widely used method for preventing hydrate plugs in gas pipelines.The method is based on injecting a single phase AC current directthrough the gas steel pipe and back through a cable strapped on the topof the pipe. This method can limit the use of inhibitors, such asmethanol. The conventional way to create a single phase power supplysuitable for the load (pipeline), is to use a 3 phase transformer with abalancing/compensating circuit on the transformer secondary windings.The balancing/compensating circuit has two capacitors and one inductorconnected between the three different phases. The transformer will besubjected to a symmetrical three phase load with power factor close to1, if these three impedances are properly matched to the pipelineimpedance.

Do to the mentioned properties of the DEH system, the balancing circuithas drawbacks:

-   -   Complex fine tuning of the circuit needs to be done during        commissioning.    -   High short circuit levels are seen at the single phase source        terminals (e.g., danger of damage to the pipeline by burning        hole in the pipeline in case of short circuit)    -   Transformer with tapping is needed to change the heating power.    -   Only the grid frequency is available in all modes

SUMMARY AND DESCRIPTION

There may be a need for an arrangement for providing an AC current to aload and for a pipeline heating arrangement, wherein the above mentioneddisadvantages are reduced, such as where switch-on transients arereduced.

Further there may be a need for an arrangement, in particular a powerelectronic unit, to heat up subsea gas pipes during production shutdowns and tail gas production without use of an output transformerconnected after the power electronic unit.

This need may be met by a power electronic circuit for DEH. The circuitwill have the same advantages as the state of the art technical solutionand reduces some of the aforementioned disadvantages.

According to an embodiment, an arrangement provides an AC current(alternating electrical current) to a load (e.g., a pipe, a pipe system,a tube for transporting oil or gas) for direct electrical heating (e.g.,of the pipe), wherein the arrangement is adapted to be used as a directelectrical heating power supply. Thereby, the arrangement includes a(just one or two, three or even more) AC-DC-AC converter cell (e.g.,controllable power electronics components such as controllableswitches), the converter cell having at least two converter inputterminals (e.g., exactly two or three) connected to at least twotransformer output terminals, the converter cell having a firstconverter output terminal and a second converter output terminal (e.g.,for outputting the AC voltage (e.g., having a frequency different fromthe frequency of the AC voltage supplied to the transformer) forsupplying energy to the load), wherein the first converter cell outputterminal is adapted to be connected to the load, wherein at all threeinput terminals of the transformer, a symmetrical load is achieved byappropriately controlling the AC-DC-AC converter cell.

Thereby, the power output of the arrangement may be controlled to, inparticular, adjust a heating power for heating a gas or oil pipeline.Further, the frequency of the AC voltage output by the arrangement maybe different from the AC voltage provided at the inputs of thearrangement. Thereby, heating efficiency of the pipeline may beimproved. In particular, it is not required to use a transformer withtabbing capability in order to change the heating power. Further, highshort circuit levels at the load may be reduced or even avoided.Further, at all three input phases of the transformer, a symmetricalload may be achieved by appropriately controlling the AC-DC-AC convertercell.

Further, a symmetric three phase load may be achieved on thetransformer, thereby requiring a less complex transformer for changingheating power.

According to an embodiment, the direct electrical heating power supplyincludes a transformer for transforming input voltages between threetransformer input terminals connected to three primary windings (or fouror five or six or seven or eight or even more, in particular, woundaround a ferromagnetic material having high magnetic permeability) tothree transformer output voltages at three secondary winding portions(e.g., inductively coupled to respective primary windings), thetransformer having the at least two transformer output terminals (e.g.,three or six or four or five or six or seven or even more);

According to an embodiment, the AC-DC-AC converter cell includes anAC-DC or rectifier section (e.g., one or more controllable switches andassociated control circuits) having two DC output nodes and beingadapted to provide a DC voltage (e.g., a direct current voltage havingan alternating current voltage overlaid, such as ripple voltage) betweenthe two DC output nodes, when an AC voltage having an input frequency isapplied between the at least two converter input terminals; and a DC-ACsection (e.g., one or more controllable switches and associated controlcircuits) having two DC input nodes connected to the two DC output nodesof the AC-DC section and being adapted to convert a DC voltage betweenthe two DC output nodes to an AC voltage having an output frequencybetween the first converter output terminal and the second converteroutput terminal.

Thereby, the arrangement may be simplified and may be assembled fromknown components, like controllable power electronics components, suchas diodes, transistors, thyristors and the like. Further, this provisionmay simplify controllability of power output, frequency of the poweroutput and may avoid short circuits at the load.

According to an embodiment, the converter cell, such as the AC-DCsection of the at least one converter cell, includes: a firstcontrollable switch (e.g., a switch, wherein opening and closing may becontrolled, in particular by a control signal); a second controllableswitch, wherein the first controllable switch and the secondcontrollable switch are connected in series between the two DC outputnodes, wherein a first one of the at least two converter input terminalsis connected between the first controllable switch and the secondcontrollable switch.

Thereby, a controllable rectifier section may be provided. Thecontrollable rectifier section may use conventional power electronicscomponents. Thereby, controllability of the arrangement may be improvedand the arrangement may be simplified.

According to an embodiment, at least one of the first controllableswitch and the second controllable switch is a thyristor.

For example, the first controllable switch includes a thyristor and alsothe second controllable switch includes a thyristor. Opening and closingthe thyristor may be controlled by an appropriate signal supplied to thegate of the respective thyristor.

For example, the thyristors may be controlled such that the currentflowing to the DC output nodes may be at least approximately constant,such that, in particular, the voltage across the DC capacitor bankfluctuates. In particular, the voltage between the two DC output nodesmay fluctuate with a frequency corresponding to two times the outputfrequency of the AC voltage output by the converter cell. In particular,when increasing the output frequency, the heating efficiency for heatingthe pipeline may increase. Thereby, heating power and/or heatingefficiency may be controlled. Further, conventional thyristors may beused to simplify the arrangement and also to reduce the costs of thearrangement.

According to an embodiment, the converter cell, in particular the AC-DCsection, further includes a capacitor, such as a capacitor for storingan electrical charge connected in parallel to the series connection ofthe first controllable switch and the second controllable switch.

Thereby, the voltage created by current flow through the thyristors(and, for example, further controllable switches connected to otherconverter input terminals of the at least two converter input terminals)may be smoothed. In particular, the capacitor may include one or morecapacitor units connected in series and/or connected in parallel betweenthe two DC output nodes. Thereby, the rectifier section may be improved.

According to an embodiment, the converter cell, in particular the AC-DCsection, includes a third controllable switch; and a fourth controllableswitch, wherein the third controllable switch and the fourthcontrollable switch are connected in series between the two DC outputnodes, wherein a second one of the at least two converter inputterminals is connected between the third controllable switch and thefourth controllable switch. In particular, the series connection of thefirst controllable switch and the second controllable switch isconnected in parallel to the series connection of the third controllableswitch and the fourth controllable switch.

Thereby, also the input voltage provided at the second one of the atleast two converter input terminals may be rectified by the thirdcontrollable switch and the fourth controllable switch and a rectifiedvoltage may be supplied to the two DC output nodes. Thereby, the poweroutput of the arrangement may be increased.

According to an embodiment, the three secondary winding portions of thetransformer are serially conductively connected in an annular manner,such that the three secondary winding portions are connected in seriesforming a loop, wherein the at least two transformer output terminalsare formed by three transformer output terminals being provided betweenpairs of the three secondary winding portions (e.g., such that eachsecondary winding portion provides one output terminal), wherein the atleast two converter input terminals are formed by three converter inputterminals for supporting three phases, wherein the three transformeroutput terminals are connected to the three converter input terminals.

Thereby, a converter cell having three converter input terminals forsupporting three electrical phases may be provided.

In particular, the first controllable switch, the second controllableswitch, the third controllable switch and/or the fourth controllableswitch and also the fifth controllable switch and/or the sixthcontrollable switch may be replaced by diodes, which may therefore notbe controlled but may adapt a conducting state depending on the voltageapplied across the respective diode.

According to an embodiment, at least one of the third controllableswitch and the fourth controllable switch includes a thyristor.

Thereby, the arrangement may further be simplified and reduced in costs,since conventional components may be used.

According to an embodiment, the converter cell, in particular the AC-DCsection, further includes a fifth controllable switch including athyristor; a sixth controllable switch including a thyristor, whereinthe fifth controllable switch and the sixth controllable switch areconnected in series between the two DC output nodes, wherein a third oneof the three converter input terminals is connected between the fifthcontrollable switch and the sixth controllable switch, wherein, forexample, the series connection of the first controllable switch and thesecond controllable switch is connected in parallel to the seriesconnection of the fifth controllable switch and the sixth controllableswitch.

Thereby, an efficient rectifier section for supporting three phases maybe provided using conventional electronic components.

According to an embodiment, the three secondary winding portions areconductively isolated from each other (e.g., formed by three separatewires wound around ferromagnetic material), wherein the at least twotransformer output terminals are formed by three pairs of transformeroutput terminals (e.g., the transformer thus having in total six outputterminals), wherein each of the three secondary winding portionsprovides one of the three pairs of transformer output terminals (e.g.,each of the three secondary winding portions provides thus twotransformer output terminals), wherein the at least two converter inputterminals are formed by just two converter input terminals, wherein thejust two converter input terminals are connected to a pair oftransformer output terminals of the three pairs of transformer outputterminals.

In particular, in total, three converter cells each having exactly twoconverter input terminals may be connected to the six transformer outputterminals. For example, the three cells connected to the onetransformer, may be connected in series at their output terminals.Thereby, the two cells at the two ends of the series connection of thethree cells may provide two output terminals, which may be connected tothe load.

According to an embodiment, at least one of the third controllableswitch and the fourth controllable switch includes a transistor, such asa IGBT, wherein, for example, a diode is connected in parallel to thethird controllable switch and, for example, another diode is connectedin parallel to the fourth controllable switch.

Providing a transistor for the third controllable switch and/or thefourth controllable switch may improve controllability of thearrangement, in particular regarding power output and heatingefficiency.

According to an embodiment, the arrangement having the transformer withsix output terminals further includes another AC-DC-AC converter cell(e.g., configured as the AC-DC-AC converter cell as described), theother converter cell having just two other converter input terminalsconnected to another pair of transformer output terminals of the threepairs of transformer output terminals, the other converter cell havinganother first converter output terminal and another second converteroutput terminal; and still another AC-DC-AC converter cell (e.g.,configured as the AC-DC-AC converter cell as described), the still otherconverter cell having just two still other converter input terminalsconnected to still another pair of transformer output terminals of thethree pairs of transformer output terminals, the still other convertercell having still another first converter output terminal and stillanother second converter output terminal, wherein the load isconnectable between the first converter output terminal and the stillother second converter output terminal, wherein the second converteroutput terminal is connected to the first other converter outputterminal, wherein the second other converter output terminal isconnected to the still other first converter output terminal.

Thereby, a series connection of the converter cell, the other convertercell and the still other converter cell is provided for providing asingle phase output (provided by two output terminals) to the load.

According to an embodiment, a series arrangement includes at least afirst and a last series connected arrangement as described above.

By providing a series arrangement, an output voltage for driving theload may be (linearly) increased, such as to adapt the output voltage tothe output voltage required by the load to heat the load. Thereby, agreater flexibility may be provided.

According to an embodiment, the series arrangement further includes acompensating capacitor either connectable in series or in parallel tothe load.

By providing the capacitor, the power factor may be adjusted. Inparticular, the reactive power at the output may be compensated by thecapacitor. Thereby, the capacitor may be placed either in series or inparallel with the load. If the capacitor is in series connected to theload, the output voltage may be kept lower and the current higher forthe converter, compared to the case where the capacitor is connected inparallel with the load. If the capacitor is in parallel with the load, asmall output choke may be put on the output of the converter, to limitthe current in the capacitor bank when switching.

According to an embodiment, a pipeline heating arrangement includes anarrangement according to one of the embodiments as describe above; andan electrically conductive pipeline connected to the arrangement as aload at a first longitudinal position and a second longitudinal positionfor electrical current flow through the pipeline from the first positionto the second position for heating the pipeline.

For example, the pipeline may be provided for transporting oil or gas.The pipeline may be manufactured from a metal, such as steel. Thepipeline may have at least two terminals for connecting the outputterminals of the heating arrangement to the first position and thesecond position, respectively.

Further, the arrangement for providing an AC current to a load, mayinclude one or more gate control circuits for controlling at least oneof the controllable switches, such that the electrical power provided atthe converter output terminals, which power is supplied to the load,satisfies desired properties, for example, regarding power output orheating efficiency, frequency of the AC output voltage and the like.Further, the gate control circuits may be adapted to control, inparticular the rectifier section of the converter cell, to achieve atleast approximately homogeneous or same load to the three primarywindings of the transformer. Thereby, a load of the three primarywinding portions may be balanced. Thereby, the efficiency of thearrangement for providing an AC current may be improved.

According to an embodiment, an arrangement for providing an AC currentto a load, such as for direct electrical heating, includes: atransformer for transforming input voltages between three transformerinput terminals connected to three primary windings to three transformeroutput voltages between three pairs of transformer output terminals,each pair of transformer output terminals being connected to arespective secondary winding inductively coupled to one of the threeprimary windings; a first, a second and a third AC-DC-AC converter cell,each cell having two converter input terminals connected to a pair ofthe transformer output terminals, each cell having a first converteroutput terminal and a second converter output terminal, wherein the loadis connectable between the first converter output terminal of the firstconverter cell and the second converter output terminal of the thirdconverter cell, wherein the second converter output terminal of thefirst converter cell is connected to the first converter output terminalof the second converter cell and wherein the second converter outputterminal of the second converter cell is connected to the firstconverter output terminal of the third converter cell.

According to an embodiment, a converter is employed in the DEH powersupply which employs power electronics for conversion (e.g. Thyristorsor IGBTs) and symmetrisation providing a similar function as aconventional symmetrisation unit as disclosed in WO 2010/031626. Usingthe converter, in particular the converters intermediate DC link, thesingle phase load of the pipeline section is transformed into asymmetric three phase load on the transformer. Further, a less complextransformer for changing the heating power may be provided, reducedshort circuit currents at the single phase load.

A further embodiment provides a direct electrical heating power supply.The direct electrical heating power supply includes a converterincluding at least one AC-DC-AC converter cell, the converter cellhaving an AC-DC section, a DC link and a DC-AC section. The directelectrical heating power supply further includes a first, a second and athird AC converter input, wherein the first converter input isconnectable to a first secondary winding of a three phase transformer,the second converter input is connectable to a second secondary windingof the three phase transformer, and the third converter input isconnectable to a third secondary winding of the three phase transformer,and an AC converter output connectable to an electrically conductivepipeline section forming a single phase load for direct electricalheating of the pipeline section. The converter is configured supply ACelectric power to the single phase load and to distribute the loadequally between the first, the second and the third AC converter inputs.

In an embodiment of the direct electrical heating power supply, theconverter may include at least three power cells, one power cellproviding the first AC converter input, one power cell providing thesecond AC converter input and one power cell providing the third ACconverter input. The output of the at least three power cells may beconnected in series to provide the AC converter output. The convertercan be configured to synchronize the generation of an AC voltage by theDC-AC sections of the at least three power cells to provide a singlephase AC output at the AC converter output. For this purpose, acontroller may be provided which controls the synchronization of theDC-AC sections of the at least three power cells.

In a further embodiment of the direct electrical heating power supply,the power cell may include three AC inputs which provide the first, thesecond and the third AC converter inputs. The AC-DC section of theconverter cell can be configured to convert a three phase AC voltagereceived on the first, the second and the third AC converter inputs,such as from the transformer's secondaries, into a common DC voltage onthe DC link. Thereby, the load on the first, the second and the third ACconverter inputs may be symmetrized.

In other embodiments of the direct electrical heating power supply, theconverter may be provided by the arrangement for providing an AC currentin any of the above outlined embodiments and configurations.

The aspects defined above and further aspects of the present inventionare apparent from the examples of embodiment to be described hereinafterand are explained with reference to the examples of embodiment. Theinvention will be described in more detail hereinafter with reference toexamples of embodiment but to which the invention is not limited.

The scope of the present invention is defined solely by the appendedclaims and is not affected to any degree by the statements within thissummary.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates an arrangement for providing an ACcurrent to a load according to an embodiment;

FIG. 2 schematically illustrates an arrangement for providing an ACcurrent to a load including three AC-DC-AC converter cells, such as theconverter cell illustrated in FIG. 1 , according to an embodiment;

FIG. 3 schematically illustrates a pipeline heating arrangement forproviding an AC current to a pipeline for heating the pipeline accordingto an embodiment, wherein the arrangement includes five seriouslyconnected arrangement as illustrated in FIG. 2 ;

FIG. 4 schematically illustrates an arrangement for providing an ACcurrent to a load according to an embodiment;

FIG. 5 schematically illustrates an arrangement for providing an ACcurrent to a load according to a still other embodiment;

FIG. 6 and FIG. 7 schematically illustrate heating arrangements forproviding an AC current to a pipeline including a plurality of AC-DC-ACconverter cells according to embodiments; and

FIG. 8 schematically illustrates a pipeline heating arrangementaccording to an embodiment, where the arrangement includes pluralAC-DC-AC converter cells.

DETAILED DESCRIPTION

The illustrations in the drawings are in schematic form. It is notedthat in different figures, similar or identical elements may be providedwith the same reference signs or with reference signs, which aredifferent from the corresponding reference signs only within the firstdigit.

FIG. 1 schematically illustrates an arrangement 100 for providing an ACcurrent to a load, wherein the arrangement 100 includes a transformerportion 101, an AC-DC section 103, a capacitor arrangement or DC-link105 and a DC-AC section 107. The arrangement 100 can form a directelectrical heating (DEH) power supply, and the AC-DC section 103, theDC-link 105 and the DC-AC section 107 can form a power cell 133 of aconverter.

The transformer portion 101 includes a secondary winding portion 109,110, which is inductively coupled to a not illustrated primary windingof a not completely illustrated transformer for transforming an AC inputvoltage to a higher AC output voltage. The portion 101 represents only aportion of a transformer, the complete transformer having threetransformer input terminals connected to three primary windings, whereineach of the three primary windings is inductively coupled to acorresponding secondary winding portion 109, 110.

The secondary winding 109, 110 is here illustrated as including twosections 109, 110, but may also include only one section, in particularincluding a wire wound around a ferromagnetic material. Optionally, thesecondary winding portion 109, 110 may include a capacitor 112 connectedbetween the transformer output terminals 111, 113. The portion 109 isconfigured as an inductance representing a secondary winding of atransformer, while the portion 110 represents an additional inductor.The capacitor 112 is a filter capacitor. The choke and the capacitor 112is not absolutely needed and may be omitted in other embodiments.

The transformer portion 101 has two transformer output terminals 111 and113.

The AC-DC section 103 includes a first controllable switch 115implemented as a thyristor; a second controllable switch 117, alsoimplemented as a thyristor, wherein the first of the transformer outputterminals 111 is connected between the thyristors 115, 117, which areconnected in series between two DC output nodes 119, 121.

The second output terminal 113 of the two transformer output terminalsis connected between a third controllable switch 123 and a fourthcontrollable switch 125, which are connected in series between the twoDC output nodes 119, 121. The third controllable switch and the fourthcontrollable switch 123, 125 are implemented each as isolated gatebipolar transistor (IGBT). In parallel to the IGBTs 123, 125, diodes 127are connected.

The capacitor section 105 includes two series connections of capacitors129, wherein the two series connections of capacitors 129 are connectedin parallel between the two DC output nodes 119, 121.

The DC-AC section 107 includes four transistors (in particular IGBTs)131, wherein a series connection of two IGBTs 131 is connected betweenthe two DC output nodes 119, 121. In parallel to the IGBTs 131, diodes127 are arranged. Two series connections of two IGBTs 131 each areconnected in parallel.

The AC-DC section 103, the capacitor section 105, and the DC-AC section107 together form an AC-DC-AC converter cell 133. The AC-DC-AC convertercell 133 has a first converter output terminal 135 connected between apair of serially connected IGBTs 131 and a second converter outputterminal 137 connected between another pair of serially connected IGBTs131.

In particular, FIG. 1 illustrates a converter cell topology with asingle phase, switch mode rectifier 103.

The input transformer has one common primary winding and one secondarywinding 109, 110 for each cell 133. Each cell 133 has a single phaseinput 111, 113. Therefore the cell converter should have a multiple of 3cells in order to give symmetrical impact on the mains. The input IGBT's123, 125 are pulse width modulation (PWM) controlled, and an inputfilter may be needed (e.g., similar to an Active Front End on a motordrive).

Using thyristors 115, 117 instead of diodes in the other phase 111 givesthe possibility of soft start and cell isolation in the case ofbreakdown on the cell.

Since this rectifier section 103 produces a pulsating power to thecapacitor section or DC link 105 with the double mains frequency, the DCcapacitor 105 is capable of withstanding the sum of the pulsating powerfrom the inverter and the rectifier.

FIG. 2 schematically illustrates an arrangement 200 for providing an ACcurrent to a load according to an embodiment. The arrangement includesthree converter cells 133 a, 133 b, 133 c, as illustrated in FIG. 1 .

The arrangement 200 illustrated in FIG. 2 includes a transformer 240having three transformer input terminals 241, 242, 243. The threetransformer input terminals 241, 242, 243 are connected to three primarywindings 244. Thereby, the windings 244 are connected in series. Theprimary windings 244 include wires wound around ferromagnetic materialcomprised in the transformer 240.

Inductively coupled to the three primary windings 244 are secondarywinding portions 109, wherein each secondary winding portion 109 has awire wound around ferromagnetic material and inductively coupled to arespective primary winding 244. The secondary windings 109 are isolatedfrom each other. Thereby, the transformer 240 has in total sixtransformer output terminals, each of the three secondary windingportion providing two terminals 111, 113.

The arrangement 200 for proving an AC current to a load further includesthree AC-DC-AC converter cells 133 (denoted as 133 a, 133 b, 133 c).Each of the converter cells 133 receives two of the six transformeroutput terminals 111, 113. Each cell 133 receives the output terminals111, 113 belonging to a particular secondary winding 109. The cells 133a, 133 b, 133 c illustrated in FIG. 2 are constructed and configured asthe cell 133 illustrated in FIG. 1 .

Each cell 133 has two converter cell output terminals 135 and 137. Ascan be taken from FIG. 2 , the first converter cell output terminal 135of the first converter cell 133 a is connectible to a load, and also thesecond converter cell output terminal 137 of the third converter cell133 c is connectible to a load, in order to provide a single phaseoutput to a load, such as a pipeline.

The second converter cell output terminal 137 of the first cell 133 a isconnected with the first converter output terminal 135 of the secondcell 133 b. The second converter cell output terminal 137 of the secondconverter cell 133 b is connected to the first converter cell outputterminal 135 of the third converter cell 133 c. Thereby, the cells 133a, 133 b and 133 c are connected in series.

According to an embodiment, a master control system (not illustrated) isprovided. The master control system may, for example, controlsynchronization of the three single phase converters 133 a, 133 b, 133 cillustrated of FIG. 2 . The master control system may provide pulsewidth modulation signals to semiconductor switches in the converters inorder to achieve a symmetrical input load on the three transformer inputterminals 241, 242, 243.

FIG. 3 schematically illustrates a pipeline heating arrangement 300including five arrangements 200 illustrated in FIG. 2 . Each arrangement200 for providing an AC current to a load 350 includes three transformerinput terminals 241, 242 and 243, which are connected to an energysupply providing the electric energy in three electrical phases 351,352, 353.

As can be taken from FIG. 3 , the arrangements 200 illustrated in FIG. 3, are connected in series. In particular the respective second outputterminal of the arrangements 200 (i.e., the output terminal 138) isconnected to a respective first output terminal (i.e., the outputterminal 136) of the next arrangement 200 in the series. The firstoutput terminal 136 of the first arrangement 200 and the second outputterminal 138 of the last arrangement 200 are connected to a compensatingcapacitor 355, which is connected to a first position 357 and a secondposition 359 of a pipeline 350. Thereby, the arrangement 300 provides anAC current through the pipeline 350 flowing from the first position 357to the second position 359.

FIG. 4 schematically illustrates an arrangement 400 for providing an ACcurrent to a load. The arrangement 400 includes a transformer portion401, an AC-DC section 403, a capacitor section or DC-link 405 and aDC-AC section 407. The arrangement 400 can form a direct electricalheating (DEH) power supply, and the AC-DC section 403, the DC-link 405and the DC-AC section 407 can form a power cell 433 of a converter.

In contrast to the embodiment illustrated in FIG. 1 , the arrangement400 includes a transformer portion 401, which includes three secondarywindings 409. The three secondary windings 409 are connected in series,wherein each of the secondary winding portions 409 is inductivelycoupled to a respective primary winding of a transformer, such as thetransformer 240 illustrated in FIG. 2 .

Each of the secondary winding portions 409 provides a voltage to thethree converter cell input terminals 411, 412 and 413. Thus, theconverter cell 433 includes three converter cell input terminals 411,412, 413 to support three phases.

The arrangement 400 further includes a first thyristor 415 and a secondthyristor 417, which are connected in series between two DC output nodes419, 421. The first converter cell input terminal 411 is connectedbetween the first thyristor 415 and the second thyristor 417.

The arrangement 400 further includes a third thyristor 423, a fourththyristor 425, a fifth thyristor 428 and a sixth thyristor 430. Thethird thyristor 423 and the fourth thyristor 425 are connected in seriesbetween the DC output nodes 419, 421, and also the fifth thyristor 428and the sixth thyristor 430 are connected in series between the two DCoutput nodes 419, 421. Further, the second converter cell input terminal412 is connected between the third thyristor 423 and the fourththyristor 425. Further, the third converter cell input terminal 413 isconnected between the fifth thyristor 428 and the sixth thyristor 430.

The capacitor section 405 includes capacitors 429 arranged and connectedas in the embodiment illustrated in FIG. 1 .

Further, the DC-AC section 407 includes IGBTs 431 and diodes 427configured and arranged as in the embodiment illustrated in FIG. 1 .

The arrangement 400 has a first converter cell output terminal 435 and asecond converter cell output terminal 437. A load, such as the pipeline350, illustrated in FIG. 3 , may be connected directly or via one ormore compensating capacitors to the converter cell output terminals 435and 437 or to a series connection of several serially connectedarrangements 400. In particular, the arrangements 200 illustrated inFIG. 3 may be replaced by the arrangements 400 illustrated in FIG. 4 orby the arrangements 500 illustrated in FIG. 5 (described below) tosupply electric energy for heating the load 350.

In particular, FIG. 4 illustrates a cell topology with a thyristorrectifier section 403. The main advantages with the thyristors are thatthe thyristors can perform a soft start, shut down locally, if the cellshould fail and can keep a constant DC voltage, if the input voltage isvarying. In addition, the thyristors can be controlled to give asymmetrical input current even if the DC link voltage has a load sidefrequency dependent ripple.

FIG. 5 schematically illustrates an arrangement 500 for providing an ACcurrent to a load. The arrangement 500 shows similarities to thearrangement 400 illustrated in FIG. 4 . Elements similar in structureand/or function in FIGS. 4 and 5 are labelled by reference signsdiffering only in the first digit.

In contrast to the arrangement 400 illustrated in FIG. 4 , thearrangement 500 illustrated in FIG. 5 does not use thyristors 415, 417,423, 425, 428, 430 connected between DC output nodes but uses diodes514. Two diodes 514 are serially connected between the DC output nodes519 and 521. Thereby, three pairs of the serially connected diodes 514are connected in parallel. Each of the three input terminals 511, 512,513 of the converter cell 533 is connected between two seriallyconnected respective diodes 514.

The arrangement 500 further includes fuses 560, which are connectedbetween the input terminals 511, 512, 513 and the secondary windingportions 509 in order to protect the arrangement 500 from over-current.

In particular, FIG. 5 illustrates a cell topology with diode rectifier503.

The input transformer has one primary winding and one secondary windingfor each cell 533. The different secondary phases are phase shifted togive a high pulse number impact on the mains. In the case of a breakdownin the cell components, the input fuses 560 will blow and isolate thefaulty cell 533, while the others can continue operating.

The rectifier 503 is a simple diode bridge and a precharging circuit onone transformer winding is provided.

The output IGBT inverter 507 in H-bridge configuration is controlled ina Pulse Width Modulation mode to give a controllable fundamental sinewave output.

The DC capacitor 505 is dimensioned to buffer the second harmonicpulsating power to a single-phase system with a limited voltage ripple.A short circuit contactor at the output will in case of cell failureisolate and short the faulty cell and allow the rest of the converteroperate with reduced peak power capacity.

FIG. 6 schematically illustrates another pipeline heating arrangement600 including a number of converter cells, such as cells 100, 400 or 500illustrated in FIG. 1, 4 or 5 . A transformer 640 includes, for example,a number of six secondary winding portions 609. The secondary windingportions 609 provide either two phases or three phases to each of thecells 633. The cells 633 are serially connected to provide anappropriate voltage to the load 650, wherein the capacitor 655 acts as acompensating capacitor.

In particular, FIG. 6 illustrates a 6-cell converter with seriescapacitor compensation.

In this configuration, the load reactance is balanced with a seriesconnected capacitor 655. When tuned correctly, the load impedance seenfrom the converter is resistive, and the converter output components areused to produce active power only.

The transformer 640 has to be specifically made for the cell converterwith separate secondary windings, with medium volt insulation, for eachcell. If necessary, transformer 640 may be divided into 2 or 3 separateunits. A converter with 6 series connected blocks will be capable of 4.1kV and 1600 A (6.6 MW).

FIG. 7 illustrates a pipe heating arrangement 700 according to anembodiment. Three transformers 740 are employed, each transformer 740providing electric energy to six secondary winding portions 709. Eachsecondary winding portion 709 is connected via two or three terminals toa cell 733, such as cells 133, 433, 533 illustrated in FIGS. 1, 4, 5 ,respectively. Since the arrangement 700 provides a three times highervoltage compared to the arrangement 600 illustrated in FIG. 6 , acompensating capacitor 655 may be avoided.

It is possible to avoid the series connected capacitor by using aconverter for full voltage and full current. Since each cell 733 maygive, for example, a maximum of 690 V AC, 18 cells may be seriesconnected to give 12 kV. Instead of the tree separate transformers shownon FIG. 7 , a single transformer with 18 secondary windings could alsobe used.

In this case, the cell is made for low cos φ, with a transformer andrectifier part rated for 300 kW pr cell. The inverter and DC-linkcapacitor is to be rated for 1600 A as for the capacitor compensatedsystem.

FIG. 8 schematically illustrates a pipeline heating arrangement 800 forheating a pipeline 850 by direct electrical heating using a seriesconnection of cells 833, such as cells 133, 433 or 533 illustrated inFIG. 1, 4 or 5 , respectively.

The cells 833 are powered by not illustrated transformers. Thethyristors 115, 117, 415, 417, 423, 425, 428, 430 illustrated in FIG. 4, and the IGBT 123, 125 illustrated in FIG. 1 are controlled regardingtheir conductivities by a pulse with modulation control 860, whichsupplies control signals to corresponding gates of the thyristors and/orIGBTs. Further, the pipeline heating arrangement 800 includes ameasuring system 861 for measuring voltage and/or current flowingthrough the pipeline 850 or between different positions of the pipelineor the whole system. Further, the arrangement 800 includes a feedbacksystem 863, which receives measurement data of the measurement system861 and controls the pulse with modulation control. In particular, thecontrol is via a closed loop control 865 using set point information 867for controlling the pulse with modulation control 860, in order toachieve desired heating power, desired voltage or desired current flowthrough the load 850. Thereby, the load is monitored by a monitoringsystem 869.

The voltage and current in the power circuit is transformed to signallevel and used as feedback to the closed loop controller. The samesignals are used for load circuit monitoring. The controller can be runin sine wave pulse pattern mode (no AC feedback) or in sine wave currentmode (AC current feedback). In sine wave current mode, the short circuitcurrent will be equal to the actual load current.

A special frequency controller part tunes the frequency to give a powerfactor better than 0.95 on the cell converter part before thecompensating capacitor. Other control functions like constant poweroutput and power input limit may be added according to demands. Theoutput of the closed loop controller 865 is the PWM signal to the cellsthrough optical fibre.

The load circuit impedance is monitored and checked against limits.Warnings and stop signal are given when limits are exceeded. The coolingsystem is monitored with respect to water level, water conductivity,water flow, and water temperature. Transformer input current andtemperature is monitored.

Each cell 833 may have its own intelligence in order to minimize thecommunication on the optical fiber. Auxiliary power and phaseinformation may be picked up from the 3 phase input power side. Thethyristor rectifier is automatically controlled by its own intelligencewhich takes care of: soft start, constant DC capacitor voltage, inputcurrent symmetry, shutdown at short circuit, and overload indication.The inverter part will take its control signal from the fiber optics.

On each cell 833, there may be a monitoring function for: heat sinktemperature, DC capacitor voltage, thyristor current symmetry, possiblethyristor fault, and IGBT switching function. The monitoring status istransferred through a separate optic fiber from each cell to a centralunit.

Most power components, like power semiconductors, DC capacitors, filterinductors and resistors, in the cell are water cooled with de-ionisedwater. Therefore, the power density in the cell may be high, and themargins in current rating of the components are kept low.

The DC capacitor may be an Aluminium Electrolytic Capacitor, designedfor base plate cooling.

When using diodes instead of thyristors, the DC voltage will varyaccording to the input voltage. The rated inverter voltage may have tobe reduced in order to keep the maximum DC working voltage below maximumrating of the DC capacitor. Therefore, the cell has to be derated with afactor proportional to the voltage variation range. It is convenientthat the input fuses are the standard 690 V type. The transformer outputvoltage should therefore not exceed 700 V.

The compensating capacitor may be connected in series with the cable inorder to reduce the voltage seen from the converter. When the capacitoris tuned to give the same reactance as the cable at the operatingfrequency, the converter will work at a power factor close to unity, andthe stress and loss in the converter components are minimized. If thesystem is to be used with variable frequency or on different cables(pipe lines), the capacitor value is varied accordingly.

Advantages of the cell converter system according to embodiments may be:

-   -   The output power and output frequency is continuously variable        (compensating capacitor is tuned to frequency).    -   The system is tolerant to harmonics on the mains.    -   The converter cell is standardized, cells can be stacked up to        the necessary power or voltage.    -   The losses of the cells are dissipated to water, the air        condition capacity can be reduced.    -   High reliability due to redundancy, possibility to bypass one        cell on the case of cell failure.    -   Possible to use the same converter on different pipes (easy        power regulation).    -   Rapid electronic switch-off in the case of load-cable breakdown.

It may be possible to implement power setting without steps and softstart.

By using power electronics, it is possible to vary the output frequency.A higher frequency than 50 Hz or 60 Hz may improve the power efficiencyon the heating system. If the output frequency is to be varied, thecompensating capacitor is varied accordingly.

The state of the art system is tuned to match the specific pipe lineimpedance when installed. The power electronic system can be used ondifferent pipe lines that demand approximately the same energy forheating. If the reactances of the different pipe lines are different,the compensating capacitor may be retuned, or different frequencies forthe different pipe lines may be used.

This new power electronic circuit for DEH according to an embodiment mayfit in the available room on an oil or gas platforms leg 8 m×3 m and maybe divided in units small enough to be hoisted down through a hatch inthe floor with the size 2.35 m×1.65 m. As an alternative, space on thepipe deck may also be used with a container solution.

The modularity of the cell converter may enable a redundant system wherea cell may be bypassed in the case of breakdown without the loss ofheating power.

The arrangement for a DEH pipeline heating system may perform threedifferent tasks: Convert 3 phase power to single phase power; Provideoptimum output frequency and power for the load; and Compensate for thelow power factor of the load, which may approximately be 0.25.

The DC current link converter using thyristors may have a unit powerrating of 2.5 MW with an operating AC voltage of 1500 V. Several unitsmay easily be paralleled, but are not so easily series connected. Anoutput transformer may be used to match the impedance in the loadcircuit.

Several DC voltage link converters using IGBT's may be series connectedfor power and voltage increase into a multi cell converter. Thisconverter can work with series capacitor compensation, and an outputtransformer may be omitted.

It should be noted that the term “comprising” does not exclude otherelements or steps and “a” or “an” does not exclude a plurality. Alsoelements described in association with different embodiments may becombined. It should also be noted that reference signs in the claimsshould not be construed as limiting the scope of the claims.

It is to be understood that the elements and features recited in theappended claims may be combined in different ways to produce new claimsthat likewise fall within the scope of the present invention. Thus,whereas the dependent claims appended below depend from only a singleindependent or dependent claim, it is to be understood that thesedependent claims can, alternatively, be made to depend in thealternative from any preceding or following claim, whether independentor dependent, and that such new combinations are to be understood asforming a part of the present specification.

While the present invention has been described above by reference tovarious embodiments, it should be understood that many changes andmodifications can be made to the described embodiments. It is thereforeintended that the foregoing description be regarded as illustrativerather than limiting, and that it be understood that all equivalentsand/or combinations of embodiments are intended to be included in thisdescription.

I claim:
 1. A direct electrical heating power supply comprising: aconverter comprising at least one AC-DC-AC converter cell, the at leastone AC-DC-AC converter cell having an AC-DC section, a DC link and aDC-AC section, a first, a second and a third converter AC input, whereinthe first converter AC input is connectable to a first secondary windingof a three phase transformer, the second converter AC input isconnectable to a second secondary winding of the three phasetransformer, and the third converter AC input is connectable to a thirdsecondary winding of the three phase transformer, a converter AC outputconnectable to an electrically conductive pipeline section forming asingle phase load for direct electrical heating of the pipeline section,wherein the converter is configured to supply AC electric power to thesingle phase load and to distribute the load equally between the first,the second and the third converter AC inputs, wherein an outputfrequency of the AC electric power is tuned to a reactance of thepipeline section.
 2. The direct electrical heating power supplyaccording to claim 1, wherein the at least one AC-DC-AC converter cellcomprises at least three AC-DC-AC converter cells, a first AC-DC-ACconverter cell providing the first converter AC input, a second AC-DC-ACconverter cell providing the second converter AC input and a thirdAC-DC-AC converter providing the third converter AC input, wherein ACoutputs of the at least three AC-DC-AC converter cells are connected inseries to provide the converter AC output, the at least three AC-DC-ACconverter cells being configured to synchronize the generation of an ACvoltage by DC-AC sections of the at least three AC-DC-AC converter cellsto provide a single phase AC output at the converter AC output.
 3. Thedirect electrical heating power supply according to claim 1, wherein theat least one AC-DC-AC converter cell comprises three AC inputs providingthe first, the second and the third converter AC inputs, wherein theAC-DC section of the at least one AC-DC-AC converter cell is configuredto convert a three phase AC voltage received on the first, the secondand the third converter AC inputs into a common DC voltage on a DC link,thereby symmetrising the load on the first, the second and the thirdconverter AC inputs.
 4. The direct electric heating power supplyaccording to claim 1, wherein the at least one AC-DC-AC converter cellcomprises a first converter output terminal and a second converteroutput terminal, wherein the first converter output terminal isconnectable with the load, wherein at all three inputs of a three phasetransformer, a symmetrical load is achievable by controlling the atleast one AC-DC-AC converter cell.